Institute for Horticultural Production Systems, Leibniz-University Hannover, Hannover, Germany.
PLoS One. 2019 Jul 31;14(7):e0219794. doi: 10.1371/journal.pone.0219794. eCollection 2019.
Rain cracking severely affects the commercial production of many fleshy-fruit species, including of sweet cherries. The objectives were to investigate how the gaping macroscopic cracks (macrocracks) of a rain-cracked fruit can develop from microscopic cracks in the cuticle (microcracks). Incubating fruit in deionized water is well known to cause significant macrocracking. We found that after a lag phase of 2 h, the numbers and lengths of macrocracks increased. Macrocrack number approached an asymptote at 12 h, whereas macrocrack length continued to increase. The rate of macrocrack propagation (extension at the crack tip) was initially 10.8 mm h-1 but then decreased to a near-constant 0.5 mm h-1. Light microscopy revealed three characteristic zones along a developing macrocrack. In zone I (ahead of the crack), the cuticle was intact, the epidermal cells were unbroken and their cell walls were thin. In zone II, the cuticle was fractured, the first epidermal cells died and their cell walls began to thicken (swell). In zone III, most epidermal cells had died, their cell walls were swollen and cell:cell separation began along the middle lamellae. The thickness of the anticlinal epidermal cell walls and the percentage of intact living cells along a crack were closely and negatively related. Cracks were stained by calcofluor white, but there was no binding of monoclonal antibodies (mAbs) specific for hemicelluloses (LM11, LM21, LM25). Strong binding was obtained with the anti-homogalacturonan mAb (LM19), indicating the presence of unesterified homogalacturonans on the crack surface. We conclude that macrocrack propagation is related to cell death and to cell wall swelling. Cell wall swelling weakens the cell:cell adhesion between neighbouring epidermal cells, which separate along their middle lamellae. The skin macrocrack propagates like a 'run' in a fine, knitted fabric.
雨水严重裂果会影响许多肉质水果物种的商业生产，包括甜樱桃。本研究的目的是调查雨裂果实中宏观裂缝（macrocracks）是如何从小的角质层裂缝（microcracks）发展而来的。将果实浸泡在去离子水中会导致明显的宏观裂缝，这是众所周知的。我们发现，在 2 小时的滞后阶段后，宏观裂缝的数量和长度增加。宏观裂缝数量在 12 小时时接近渐近线，而宏观裂缝长度继续增加。宏观裂缝扩展的速度（裂缝尖端的延伸）最初为 10.8mm/h，但随后降至接近恒定的 0.5mm/h。 光学显微镜显示，在一条发育中的宏观裂缝上有三个特征区。在 I 区（裂缝前方），角质层完整，表皮细胞未破裂，细胞壁较薄。在 II 区，角质层破裂，第一个表皮细胞死亡，细胞壁开始增厚（肿胀）。在 III 区，大多数表皮细胞已经死亡，细胞壁肿胀，细胞：细胞分离开始沿中层发生。沿着裂缝的垂周壁表皮细胞细胞壁的厚度与完整活细胞的百分比密切负相关。用 Calcofluor White 染色裂缝，但没有与半纤维素（LM11、LM21、LM25）特异性的单克隆抗体（mAbs）结合。用抗 homogalacturonan mAb（LM19）获得强烈结合，表明裂缝表面存在未酯化的 homogalacturonans。我们得出的结论是，宏观裂缝的扩展与细胞死亡和细胞壁肿胀有关。细胞壁肿胀削弱了相邻表皮细胞之间的细胞：细胞粘附，这些细胞沿着中层分离。皮肤宏观裂缝像细编织物中的“线迹”一样扩展。
